This is one of the pieces of music I love. The version I have is on a CD set called Les Introuvables de Jacqueline Du Pré, and, when I hear it, I want to dance with joy. I don't have a very musical ear and I don't know much about music, so I can't really explain why I love it so much - My favourite part is the Allegro moderato, which just jumps and leaps in my ears and makes me happy. I love its strong rythm. Then, the Adagio settles at a slower pace. And the Rondo (Allegro) picks up again, more subdued than the first part, but still with a lovely melody to it.
Anyway, as I said, I don't know much about music, so I can't really explain. Go to the Amazon site and listen to the samples. Althought they are short, they give a good sense of the piece.
Here is a link to the Wikipedia article about Joseph Haydn, but I'll confess I only glanced through it.